[PATCH 20/20] 9p: fix ->rename_sem exclusion

Al Viro
2025-01-10T02:43:07Z

9p wants to be able to build a path from given dentry to fs root and keep
it valid over a blocking operation.

->s_vfs_rename_mutex would be a natural candidate, but there are places
where we need that and where we have no way to tell if ->s_vfs_rename_mutex
is already held deeper in callchain. Moreover, it's only held for
cross-directory renames; name changes within the same directory happen
without it.

Solution:
* have d_move() done in ->rename() rather than in its caller
* maintain a 9p-private rwsem (per-filesystem)
* hold it exclusive over the relevant part of ->rename()
* hold it shared over the places where we want the path.

That almost works. FS_RENAME_DOES_D_MOVE is enough to put all d_move()
and d_exchange() calls under filesystem's control. However, there's
also __d_unalias(), which isn't covered by any of that.

If ->lookup() hits a directory inode with preexisting dentry elsewhere
(due to e.g. rename done on server behind our back), d_splice_alias()
called by ->lookup() will move/rename that alias.

An approach to fixing that would be a couple of optional methods, so that
__d_unalias() would do
if alias->d_op->d_unalias_trylock != NULL
if (!alias->d_op->d_unalias_trylock(alias))
fail (resulting in -ESTALE from lookup)
__d_move(...)
if alias->d_op->d_unalias_unlock != NULL
alias->d_unalias_unlock(alias)
where it currently does __d_move(). 9p instances would be down_write_trylock()
and up_write() of ->rename_mutex.

However, to reduce dentry_operations bloat, let's add one method instead -
->d_want_unalias(alias, true) instead of ->d_unalias_trylock(alias) and
->d_want_unalias(alias, false) instead of ->d_unalias_unlock(alias).

Another possible variant would be to hold ->rename_sem exclusive around
d_splice_alias() calls in 9p ->lookup(), but that would cause a lot of
contention on that rwsem and it's filesystem-wide, so let's not go there.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
---
Documentation/filesystems/locking.rst | 2 ++
Documentation/filesystems/vfs.rst | 19 +++++++++++++++++++
fs/9p/v9fs.h | 2 +-
fs/9p/vfs_dentry.c | 13 +++++++++++++
fs/dcache.c | 6 ++++++
include/linux/dcache.h | 1 +
6 files changed, 42 insertions(+), 1 deletion(-)

diff --git a/Documentation/filesystems/locking.rst b/Documentation/filesystems/locking.rst
index 146e7d8aa736..6e20282447a0 100644
--- a/Documentation/filesystems/locking.rst
+++ b/Documentation/filesystems/locking.rst
@@ -31,6 +31,7 @@ prototypes::
struct vfsmount *(*d_automount)(struct path *path);
int (*d_manage)(const struct path *, bool);
struct dentry *(*d_real)(struct dentry *, enum d_real_type type);
+ bool (*d_want_unalias)(const struct dentry *, bool);

locking rules:

@@ -50,6 +51,7 @@ d_dname: no no no no
d_automount: no no yes no
d_manage: no no yes (ref-walk) maybe
d_real no no yes no
+d_want_unalias yes no no no
================== =========== ======== ============== ========

inode_operations
diff --git a/Documentation/filesystems/vfs.rst b/Documentation/filesystems/vfs.rst
index 7c352ebaae98..07d4b4deb252 100644
--- a/Documentation/filesystems/vfs.rst
+++ b/Documentation/filesystems/vfs.rst
@@ -1265,6 +1265,7 @@ defined:
struct vfsmount *(*d_automount)(struct path *);
int (*d_manage)(const struct path *, bool);
struct dentry *(*d_real)(struct dentry *, enum d_real_type type);
+ bool (*d_want_unalias)(const struct dentry *, bool);
};

``d_revalidate``
@@ -1428,6 +1429,24 @@ defined:

For non-regular files, the 'dentry' argument is returned.

+``d_want_unalias``
+ if present, will be called by d_splice_alias() before and after
+ moving a preexisting attached alias. The second argument is
+ true for call before __d_move() and false for the call after.
+ Returning false on the first call prevents __d_move(), making
+ d_splice_alias() fail with -ESTALE; return value on the second
+ call is ignored.
+
+ Rationale: setting FS_RENAME_DOES_D_MOVE will prevent d_move()
+ and d_exchange() calls from the outside of filesystem methods;
+ however, it does not guarantee that attached dentries won't
+ be renamed or moved by d_splice_alias() finding a preexisting
+ alias for a directory inode. Normally we would not care;
+ however, something that wants to stabilize the entire path to
+ root over a blocking operation might need that. See 9p for one
+ (and hopefully only) example.
+
+
Each dentry has a pointer to its parent dentry, as well as a hash list
of child dentries. Child dentries are basically like files in a
directory.
diff --git a/fs/9p/v9fs.h b/fs/9p/v9fs.h
index 698c43dd5dc8..f28bc763847a 100644
--- a/fs/9p/v9fs.h
+++ b/fs/9p/v9fs.h
@@ -202,7 +202,7 @@ static inline struct v9fs_session_info *v9fs_inode2v9ses(struct inode *inode)
return inode->i_sb->s_fs_info;
}

-static inline struct v9fs_session_info *v9fs_dentry2v9ses(struct dentry *dentry)
+static inline struct v9fs_session_info *v9fs_dentry2v9ses(const struct dentry *dentry)
{
return dentry->d_sb->s_fs_info;
}
diff --git a/fs/9p/vfs_dentry.c b/fs/9p/vfs_dentry.c
index 872c1abe3295..b2222df318d0 100644
--- a/fs/9p/vfs_dentry.c
+++ b/fs/9p/vfs_dentry.c
@@ -105,14 +105,27 @@ static int v9fs_lookup_revalidate(struct inode *dir, const struct qstr *name,
return __v9fs_lookup_revalidate(dentry, flags);
}

+static bool v9fs_dentry_want_unalias(const struct dentry *dentry, bool lock)
+{
+ struct v9fs_session_info *v9ses = v9fs_dentry2v9ses(dentry);
+
+ if (lock)
+ return down_write_trylock(&v9ses->rename_sem);
+
+ up_write(&v9ses->rename_sem);
+ return true;
+}
+
const struct dentry_operations v9fs_cached_dentry_operations = {
.d_revalidate = v9fs_lookup_revalidate,
.d_weak_revalidate = __v9fs_lookup_revalidate,
.d_delete = v9fs_cached_dentry_delete,
.d_release = v9fs_dentry_release,
+ .d_want_unalias = v9fs_dentry_want_unalias,
};

const struct dentry_operations v9fs_dentry_operations = {
.d_delete = always_delete_dentry,
.d_release = v9fs_dentry_release,
+ .d_want_unalias = v9fs_dentry_want_unalias,
};
diff --git a/fs/dcache.c b/fs/dcache.c
index 7d42ca367522..efbfbc1bc5d4 100644
--- a/fs/dcache.c
+++ b/fs/dcache.c
@@ -2947,6 +2947,7 @@ static int __d_unalias(struct dentry *dentry, struct dentry *alias)
{
struct mutex *m1 = NULL;
struct rw_semaphore *m2 = NULL;
+ bool (*extra_trylock)(const struct dentry *, bool);
int ret = -ESTALE;

/* If alias and dentry share a parent, then no extra locks required */
@@ -2961,7 +2962,12 @@ static int __d_unalias(struct dentry *dentry, struct dentry *alias)
goto out_err;
m2 = &alias->d_parent->d_inode->i_rwsem;
out_unalias:
+ extra_trylock = alias->d_op->d_want_unalias;
+ if (extra_trylock && !extra_trylock(alias, true))
+ goto out_err;
__d_move(alias, dentry, false);
+ if (extra_trylock)
+ extra_trylock(alias, false);
ret = 0;
out_err:
if (m2)
diff --git a/include/linux/dcache.h b/include/linux/dcache.h
index 4a6bdadf2f29..2b33b9d04a8f 100644
--- a/include/linux/dcache.h
+++ b/include/linux/dcache.h
@@ -159,6 +159,7 @@ struct dentry_operations {
struct vfsmount *(*d_automount)(struct path *);
int (*d_manage)(const struct path *, bool);
struct dentry *(*d_real)(struct dentry *, enum d_real_type type);
+ bool (*d_want_unalias)(const struct dentry *, bool);
} ____cacheline_aligned;

/*
--
2.39.5